Initial four wheel steering systems on automotive vehicles determined the rear wheel steer angle strictly as a function of the front wheel steer angle. The initial systems did not distinguish between steady state and transient steering maneuvers. In steady state operation, the vehicle sideslip was reduced, but during transient maneuvers the vehicle sideslip generated was in a negative direction, toward the inside of a turn, causing a non-responsive feeling for the driver. Additionally, there was no improvement in generating the rotational force, yaw motion, in the direction that the vehicle was turning.
The apparent non-responsiveness is a result of the basic front wheel steer vehicle handling characteristics as compared to the four wheel steer vehicle handling characteristics. When the driver initiates a transient maneuver, the vehicle experiences both yaw and lateral acceleration. Lateral acceleration is the cornering force on the vehicle. Yaw is rotational motion about the vehicle center of gravity. In a front wheel steer vehicle, these forces occur at approximately the same time, causing the driver to feel pushed toward the outside of the turning circle. In a conventional four wheel steer vehicle, where rear wheel steer angle is a function of front wheel position only, the generation of the yaw sensation is delayed so that the lateral acceleration occurs first. This causes the driver to feel pulled into the turn as compared to being pushed away from the turn as in a two wheel steer vehicle. Such a four wheel steer system is called a proportional system because the rear wheels are positioned in proportion to the front wheels. This system has been improved by making the rear wheel position dependent not only on the front wheel position, but also on vehicle speed. This enables the rear wheel position to be tailored specifically in accordance with front wheel position and vehicle speed. As such, at higher vehicle speeds, the rear wheels will steer more in phase with the front wheels than at lower vehicle speeds for the same front wheel position, adding stability.
In more recent four wheel steer systems, it has been proposed to delay or momentarily reverse the rear steering relative to the rate of change of front steering. This quickens the generation of yaw velocity with the intent of bringing lateral acceleration and yaw velocity into phase.